Treatment of Aromatic Alkylation Feedstock

ABSTRACT

In a process and system for treatment of feed stocks comprising alkylating agent and metal salts, the metal salts are removed from the feedstock by an efficient combination of separations processes. The processes may take place in one or more stages, each stage taking place in one or more vessels. Such treatment processes may remove 99.9% or more of metal salts from a feedstock, while recovering 99.9% or more of the alkylating agent from the feedstock for use in an alkylation reaction, especially of aromatics such as toluene and benzene. Preferred alkylating agents include methanol and mixtures of carbon monoxide and hydrogen, for methylation of toluene and/or benzene. The methylation proceeds over an aluminosilicate catalyst and preferably yields para-xylene with 75% or greater selectivity.

PRIORITY CLAIM

The present application claims priority to and the benefit of U.S.Provisional Application No. 62/043,785, filed Aug. 29, 2014, thedisclosure of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to processes, systems, and apparatus for theproduction of an alkylating agent substantially free of metal salt. Inaddition, this invention relates to the production of alkylatedaromatics, particularly para-xylene, using the purified alkylatingagent.

BACKGROUND OF THE INVENTION

Alkylating agents, and particularly methylating agents, are an importantfeedstock in the production of alkyl aromatics such as xylenes byalkylation. Of the xylene isomers, para-xylene (often abbreviated PX) isof particular value for many reasons, including its use in themanufacture of terephthalic acid, which is an intermediate in themanufacture of synthetic fibers. Equilibrium mixtures of xylene isomerseither alone or in further admixture with ethylbenzene generally containonly about 24 wt % para-xylene and separation of p-xylene from suchmixtures has typically required absorption and/or multistagerefrigeration steps. Such processes have involved high operation costsand resulted in only limited yields.

Recently, an improved process of toluene methylation by methylatingagents has been developed using certain aluminosilicate zeolitemolecular sieve catalysts that, when treated and used under appropriateconditions, may exhibit high selectivity to the para-xylene isomer intoluene methylation, with per-pass toluene conversion of at least about15%. This important development has been described in numerous patentsand publications, such as U.S. Pat. Nos. 4,002,698; 4,356,338;4,423,266; 5,675,047; 5,804,690; 5,939,597; 6,028,238; 6,046,372;6,048,816; 6,156,949; 6,423,879; 6,504,072; 6,506,954; 6,538,167; and6,642,426, the entirety of each of which is incorporated herein byreference.

However, some molecular sieve catalysts that may be utilized inefficient aromatic alkylation, such as toluene methylation with highselectivity to para-xylene, may be sensitive to various contaminantsthat can “poison” the catalyst (i.e., reduce the activity of thecatalyst and/or reduce its selectivity to a desired end product).Poisoning can shorten catalyst life, requiring more frequentregeneration and/or replacement, significantly adding to the costs of anaromatic alkylation process employing the catalyst.

SUMMARY OF THE INVENTION

Systems, apparatus, and methods are provided for treating a feedstock ofalkylating agent to remove most or all metal salts therein. The treatedfeedstock may, thereafter, be used for producing alkylated aromatics andparticularly para-xylene. More particularly, the feedstock is treated inone or more stages to remove most or all metal salts present thereinbefore the feedstock is contacted with an aluminosilicate zeolitecatalyst under conditions sufficient to produce an alkylated aromaticcompound. Removing most or all metal salts from the alkylating agentprior to contacting the aluminosilicate zeolite catalyst reduces orprevents catalyst poisoning and thereby extends catalyst life.

Accordingly, one aspect of the present invention provides a process fortreating a feedstock, in which a feedstock comprising alkylating agentand metal salt is separated in one or more stages to provide at leastone alkylating agent-rich feed stream and a metal salt-rich discharge.The separation can take place in a single stage using a dividing-wallcolumn. Alternatively or in addition, the separation can includemultiple stages. The separation may include separating the feedstockinto a first vaporized alkylating agent-rich feed stream and a metalsalt-rich liquid blowdown. In some cases, the blowdown is furtherseparated in one or more additional stages into at least an additionalalkylating agent-rich vapor stream and a metal salt-rich liquiddischarge. Alternatively, or in addition, the blowdown may be divertedfor use in other processes not sensitive to metal salt content.

At least a portion of the alkylating agent-rich vapor feed stream(s) arereacted with one or more aromatic compounds in the presence of analuminosilicate zeolite catalyst. Reaction of aromatic compounds andalkylating agent in the presence of the aluminosilicate zeolite catalystyields alkylated aromatic products. In a preferred example, the aromaticcompound is toluene and/or benzene and the alkylating agent is methanoland the reaction of in the presence of the aluminosilicate zeolitecatalyst yields para-xylene.

In a further aspect, the present invention provides systems andapparatus for carrying out the various processes provided herein. Suchsystems and/or apparatus include a separation system for separating afeedstock comprising alkylating agent and metal salt into an alkylatingagent-rich feed stream and a metal salt-rich discharge. The separationsystem may contain one or more vessels for separation in one or morestages. For example, a separations unit is provided comprising adividing-wall column comprising at least a desalting zone and analkylating agent recovery zone. Alternatively, multiple separationsunits may be provided, each separation unit being selected from amongone or a combination of: a vapor/liquid separator such as a blowdowndrum, a desalting unit, and a dividing wall column. A desalting unit maybe a sorption separations unit such as an adsorption, absorption, and/orion-exchange separations unit. The invention may additionally include anaromatic alkylation unit for alkylating one or more aromatic compoundswith at least a portion of the alkylating agent-rich vapor feed streamprovided by the separation system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a process for treating feedstock for theproduction of alkyl aromatics according to a first embodiment of theinvention.

FIG. 2 is a flow diagram of a process for treating feedstock for theproduction of alkyl aromatics according to a second embodiment of theinvention.

FIG. 3 is a flow diagram of a process for treating feedstock for theproduction of alkyl aromatics according to a modification of the secondembodiment of the invention.

FIG. 4 is a graph illustrating operation of a process and system fortreating feedstock according to various embodiments of the invention.

FIG. 5 is a schematic diagram illustrating an apparatus for treatingfeedstock for the production of alkyl aromatics according to a furtherembodiment of the invention.

FIG. 6 is a schematic diagram illustrating a simulated example systemfor treating feedstock for the production of alkyl aromatics designedaccording to some embodiments of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention describes processes, systems, and apparatus fortreating feedstock containing alkylating agent and metal salts, so as toremove most or all of the metal salts from the feedstock. The treatedfeedstock may be used for alkylating aromatics in the presence of analuminosilicate zeolite catalyst, preferably methylation of benzeneand/or toluene to form para-xylene. The treated feedstock advantageouslymay contain little or substantially no metal salt by the time itcontacts the aluminosilicate zeolite catalyst, thereby reducing oravoiding catalyst poisoning by the metal salt and accordingly extendingcatalyst life.

A process according to some embodiments may include separating afeedstock comprising an alkylating agent and metal salt into at least analkylating agent-rich feed stream and a metal salt-rich discharge. Whena stream is described herein as being “rich,” “rich in,” or “enriched”in a specified species, it is meant that the wt % of the specifiedspecies in that stream is enriched relative to the feed stream prior toseparation. When a stream is described as being “depleted” in aspecified species, it is meant that the wt % of the specified species inthat stream is reduced relative to the feed stream prior to separation.Thus, a post-separation stream is “alkylating agent-rich” when the wt %of alkylating agent in that stream is enriched relative to the wt % ofalkylating agent in the corresponding stream prior to separation.

The alkylating agent-rich feed stream in some embodiments may further bereacted with one or more aromatic compounds in the presence of analuminosilicate zeolite catalyst so as to carry out alkylation of atleast some of the one or more aromatic compounds. In such embodiments,it is particularly important that the alkylating agent-rich feed streamhave little to no metal salts so as to avoid or reduce poisoning of thealuminosilicate zeolite catalyst.

Aluminosilicate Zeolite Catalysts and Metal Salt Poisoning

Any method known in the art for adding alkyl groups to a phenyl ring canbe used in the alkylation step of certain embodiments. In particular,the alkylation step may comprise methylation of a phenyl ring, such asmethylation of benzene and/or toluene to form para-xylene. Thus, certainpreferred embodiments include reaction (e.g., alkylation, andparticularly methylation) in the presence of a highly para-selectivealuminosilicate zeolite catalyst, such as that employed in U.S. Pat.Nos. 6,423,879 and 6,504,072, the entire contents of which areincorporated herein by reference. Such a catalyst comprises a molecularsieve having a Diffusion Parameter for 2,2-dimethylbutane of about0.1-15 sec⁻¹, such as 0.5-10 sec⁻¹, when measured at a temperature of120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa). As usedherein, the Diffusion Parameter of a particular porous crystallinematerial is defined as D/r²×10⁶, wherein D is the diffusion coefficient(cm²/sec) and r is the crystal radius (cm). The required diffusionparameters can be derived from sorption measurements provided theassumption is made that the plane sheet model describes the diffusionprocess. Thus, for a given sorbate loading Q, the value Q/Q_(∞), whereQ_(∞) is the equilibrium sorbate loading, is mathematically related to(Dt/r²)^(1/2) where t is the time (sec) required to reach the sorbateloading Q. Graphical solutions for the plane sheet model are given by J.Crank in “The Mathematics of Diffusion”, Oxford University Press, ElyHouse, London, 1967, the entire contents of which are incorporatedherein by reference.

A molecular sieve employed in a para-selective methylation processaccording to some embodiments is normally a medium-pore sizealuminosilicate zeolite. Medium pore zeolites are generally defined asthose having a pore size of about 5 to about 7 Angstroms, such that thezeolite freely sorbs molecules such as n-hexane, 3-methylpentane,benzene, and p-xylene. Another common definition for medium porezeolites involves the Constraint Index test which is described in U.S.Pat. No. 4,016,218, which is incorporated herein by reference. In thiscase, medium pore zeolites have a Constraint Index of about 1-12, asmeasured on the zeolite alone without the introduction of oxidemodifiers and prior to any steaming to adjust the diffusivity of thecatalyst. Particular examples of suitable medium pore zeolites includeZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-48, and MCM-22, withZSM-5 and ZSM-11 being particularly preferred.

The medium pore zeolites described above are particularly effective formethylation processes of certain embodiments since the size and shape oftheir pores favor the production of p-xylene over the other xyleneisomers. Conventional forms of these zeolites have Diffusion Parametervalues in excess of the 0.1-15 sec⁻¹ range referred to above. However,the required diffusivity for the catalyst can be achieved by severelysteaming the catalyst so as to effect a controlled reduction in themicropore volume of the catalyst to not less than 50%, and preferably50-90%, of that of the unsteamed catalyst. Reduction in micropore volumeis derived by measuring the n-hexane adsorption capacity of thecatalyst, before and after steaming, at 90° C. and 75 torr n-hexanepressure.

Steaming of the zeolite is effected at a temperature of at least about950° C., preferably about 950° C. to about 1075° C., and most preferablyabout 1000° C. to about 1050° C. for about 10 minutes to about 10 hours,preferably from 30 minutes to 5 hours.

To effect the desired controlled reduction in diffusivity and microporevolume, it may be desirable to combine the zeolite, prior to steaming,with at least one oxide modifier, such as at least one oxide selectedfrom elements of Groups 2 to 4 and 13 to 16 of the Periodic Table, asnumbered according to IUPAC, “Nomenclature of Inorganic Chemistry,” at51 (2005). Most preferably, said at least one oxide modifier is selectedfrom oxides of boron, magnesium, calcium, lanthanum, and most preferablyphosphorus. In some cases, the zeolite may be combined with more thanone oxide modifier, for example a combination of phosphorus with calciumand/or magnesium, since in this way it may be possible to reduce thesteaming severity needed to achieve a target diffusivity value. In someembodiments, the total amount of oxide modifier present in the catalyst,as measured on an elemental basis, may be between about 0.05 and about20 wt %, and preferably is between about 0.1 and about 10 wt %, based onthe weight of the final catalyst.

Where the modifier includes phosphorus, incorporation of modifier intothe catalyst is conveniently achieved by the methods described in U.S.Pat. Nos. 4,356,338, 5,110,776, 5,231,064 and 5,348,643, the entiredisclosures of which are incorporated herein by reference. Treatmentwith phosphorus-containing compounds can readily be accomplished bycontacting the zeolite, either alone or in combination with a binder ormatrix material, with a solution of an appropriate phosphorus compound,followed by drying and calcining to convert the phosphorus to its oxideform. Contact with the phosphorus-containing compound is generallyconducted at a temperature of about 25° C. and about 125° C. for a timebetween about 15 minutes and about 20 hours. The concentration of thephosphorus in the contact mixture may be between about 0.01 and about 30wt %. Suitable phosphorus compounds include, but are not limited to,phosphonic, phosphinous, phosphorous and phosphoric acids, salts andesters of such acids, and phosphorous halides.

After contacting with the phosphorus-containing compound, the porouscrystalline material may be dried and calcined to convert the phosphorusto an oxide form. Calcination can be carried out in an inert atmosphereor in the presence of oxygen, for example, in air at a temperature ofabout 150° C. to 750° C., preferably about 300° C. to 500° C., for atleast 1 hour, preferably 3-5 hours. Similar techniques known in the artcan be used to incorporate other modifying oxides into the catalystemployed in the alkylation process.

When treated with oxide modifier according to any of the various mannersdescribed herein, a portion of the aluminum of the aluminosilicatezeolite catalyst may leave the zeolite's crystal structure, leavingbehind a crystalline defect and forming amorphous aluminum oxide (e.g.,aluminum phosphate where the oxide modifier comprises phosphorus),either within the zeolite channels or external to the zeolite'schannels. Unlike other phosphorous-containing molecular sieves such asALPOs (aluminophosphate molecular sieves) and SAPOs(silicoaluminophosphate molecular sieves), treated aluminosilicatezeolites of such embodiments do not include phosphorus among theirordered crystalline structure even after such treatment.

In addition to the zeolite and modifying oxide, an aluminosilicatecatalyst employed in an alkylation process such as methylation ofaromatics may include one or more binder or matrix materials resistantto the temperatures and other conditions employed in the process. Suchmaterials include active and inactive materials such as clays, silicaand/or metal oxides, such as alumina. The latter may be either naturallyoccurring or in the form of gelatinous precipitates or gels includingmixtures of silica and metal oxides. Use of a material which is active,tends to change the conversion and/or selectivity of the catalyst andhence is generally not preferred. Inactive materials suitably serve asdiluents to control the amount of conversion in a given process so thatproducts can be obtained economically and orderly without employingother means for controlling the rate of reaction. These materials may beincorporated into naturally occurring clays, e.g., bentonite and kaolin,to improve the crush strength of the catalyst under commercial operatingconditions. Said materials, i.e., clays, oxides, etc., function asbinders for the catalyst. It is desirable to provide a catalyst havinggood crush strength because in commercial use it is desirable to preventthe catalyst from breaking down into powder-like materials. These clayand/or oxide binders have been employed normally only for the purpose ofimproving the crush strength of the catalyst.

Naturally occurring clays which can be composited with the porouscrystalline material include the montmorillonite and kaolin family,which families include the subbentonites, and the kaolins commonly knownas Dixie, McNamee, Georgia and Florida clays or others in which the mainmineral constituent is halloysite, kaolinite, dickite, nacrite, oranauxite. Such clays can be used in the raw state as originally mined orinitially subjected to calcination, acid treatment or chemicalmodification.

In addition to the foregoing materials, the porous crystalline materialcan be composited with a porous matrix material such as silica-alumina,silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia,silica-titania as well as ternary compositions such assilica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesiaand silica-magnesia-zirconia.

The relative proportions of porous crystalline material and inorganicoxide matrix vary widely, with the content of the former ranging fromabout 1 to about 90% by weight and more usually, particularly when thecomposite is prepared in the form of beads, in the range of about 2 toabout 80 wt % of the composite. Preferably, the matrix materialcomprises silica or a kaolin clay.

The methylation catalyst used in various embodiments may optionally beprecoked. The precoking step may be carried out by initially loadinguncoked catalyst into the methylation reactor. Then, as the reactionproceeds, coke is deposited on the catalyst surface and thereafter maybe controlled within a desired range, typically from about 1 to about 20wt % and preferably from about 1 to about 5 wt %, by periodicregeneration through exposure to an oxygen-containing atmosphere at anelevated temperature.

Aluminosilicate zeolite catalysts of various types according to theabove description may exhibit heightened sensitivity to poisoning due tothe presence of metal salts such as sodium salts in any compoundcontacted with such catalysts. A “metal salt” as used herein includesany salt of a metal of Group 1 or Group 2 of the Periodic Table of theElements, such as a sodium salt, and further includes any salt of atransition metal, such as a copper salt. Further, as used herein,discussion of concentration and/or other content of a “metal salt” isintended to include to both the salt compound (e.g., sodium chloride,NaCl) and dissociated corresponding metal ions (e.g., Na⁺). For example,in some instances, metal salt and/or metal ions may contact thealuminosilicate zeolite and reduce the acidity of the catalyst.Catalysts according to certain embodiments may include relatively fewalumina sites (i.e., they may have high silica-alumina molar ratios,such as 250 or more in some embodiments), which impart acidity to thecatalyst. Thus, contamination of even a small number of these sites bymetal salts may have a significant impact on the lifetime of thecatalyst due to the already small number of acidity-imparting aluminasites. Moreover, metal salts present in a feedstock even in smallquantities may lead to buildup of metal ions in the catalyst over time.For instance, a feed stream having 0.32 wppm metal salt may lead tometal concentration of 8400 wppm on the aluminosilicate zeolite catalystafter 1 year under standard operating conditions. As another example, 1wppm of metal salt may lead to buildup of 1,250 wppm of metal in thecatalyst after 1 year of operation at about 1.5 weight hourly spacevelocity (WHSV) on the basis of catalyst volume. After 5 years, themetal concentration on the catalyst would be about 2,000 wppm. Removalof most or substantially all metal salts from any stream contacting thecatalyst according to some embodiments could improve catalyst life by asmuch as two, three, four, or five times. For instance, in someembodiments, aluminosilicate zeolite catalysts may be kept in continuousoperation without replacement or regeneration for at least any one of 4,5, 6, 7, 8, 9, or 10 years according to various embodiments.

Sources of Metal Salt Poisoning

In some circumstances, metal salts may be introduced to thealuminosilicate zeolite catalyst when such salts are present in a feedstream contacting the catalyst. For instance, alkylation of aromaticsover aluminosilicate zeolite catalyst takes place according to someembodiments by reaction between an alkylating agent feed stream and anaromatic feed stream. Particular embodiments involve methylation ofbenzene and/or toluene to form xylene, preferably para-xylene. Thus,certain preferred embodiments include feed stream of a methylating agentsuch as methanol, dimethyl ether, and/or a mixture of carbon monoxideand hydrogen, and feed stream of benzene and/or toluene contacting thecatalyst. It will be appreciated that feed streams according to variousembodiments may also include other methylating and/or alkylating agents,depending upon the desired alkylation. For instance, alcohols such asethanol and propanol, and halides such as methyl, ethyl, propyl, and C₄+halides may be suitable alkylating agents according to some embodiments.

Although some feed streams may include metal salts as residual compoundsdue to the method of commercial manufacture of those feeds, in manyinstances, metal salts may result from transportation. Taking methanolfeedstock for example, metal salts are generally not present in methanolas it is typically commercially produced. However, transport andparticularly ocean transport of the methanol frequently results inintroduction of at least trace amounts of metal salts. In fact, theInternational Methanol Producers and Consumers Association (IMPCA)provides a commercial specification for methanol that includes <0.32 ppmNa, as indicated by a Cl spec of <0.5 ppm (the source of Cl beingseawater). Seawater may enter methanol storage vessels on ships due tominor leaks, or due to storage tank washing between voyages withbrackish water, or the like. Other alkylating agent feed stocks may besubject to the same pitfalls at least via transportation, if not also orinstead due to their manufacture.

Therefore, depending upon the source of an alkylating agent feedstock,it may be necessary to treat that feedstock so as to remove metal saltslike sodium salts, in accordance with some embodiments of the presentinvention.

Treatment of Feedstock

Some embodiments of the present invention provide processes, methods,and/or apparatus tailored to treat alkylating agent feedstock to providea treated feedstock having the extremely low concentration of metalsalts necessary for contacting the treated feedstock with analuminosilicate zeolite catalyst.

A process according to several embodiments includes separating afeedstock comprising an alkylating agent and a metal salt into at leastan alkylating agent-rich feed stream and a metal salt-rich discharge.The process according to some embodiments may further include reactingat least a portion of the alkylating agent-rich feed stream with one ormore aromatic compounds in the presence of an aluminosilicate zeolitecatalyst.

Separating the feedstock may comprise any one or more of severalseparation steps, and each step may entail one or more stages in one ormore separations vessels. According to some embodiments, separation maycomprise a blowdown separation step, comprising separating the feedstockinto (i) an alkylating agent-rich vapor feed stream and (ii) a metalsalt-rich liquid blowdown. The blowdown separation step may take placein one or more separation stages, such as one or more heating stages.The vapor feed stream (i) may then be passed to alkylation reaction overthe aluminosilicate zeolite catalyst, while the blowdown (ii) mayaccording to some embodiments be subjected to further treatment. Furtherblowdown treatment according to such embodiments includes furtherseparation to recover alkylating agent while rejecting metal salts. Incertain embodiments, blowdown treatment includes a desalting step and analkylating agent recovery step. The desalting step and alkylating agentrecovery step may take place in separate vessels in series, or they mayboth take place in a single vessel (such as a dividing-wall column).

In certain embodiments, the first blowdown separation step may beomitted, and feedstock passed directly to desalting/alkylating agentrecovery separations, particularly in embodiments in which both thedesalting and recovery steps take place in a single vessel.

On the other hand, according to other embodiments, the blowdown may notbe subjected to additional treatment, but instead may be diverted toanother use that is not sensitive to metal salt content (such as fuelblending or the like), thereby avoiding undesirable waste of feedstockwhile maintaining integrity of the aromatic alkylation process.

Respective aspects of feedstock treatment according to these and othervarious embodiments are each discussed in greater detail below.

Blowdown Separation

Treatment of feedstock comprising alkylating agent and metal salts(e.g., as entering via line 101 in FIG. 1) may be treated initiallyaccording to some embodiments by blowdown separation. As shown in FIGS.1 and 2, this separation may be effected by a blowdown drum 110. Theblowdown drum 110 may be or may comprise any suitable unit, such as aknockout drum, distillation column, thermosyphon, or any other suitableseparation vessel(s) for separating the feedstock into an alkylatingagent-rich feed stream and a metal salt-rich blowdown, as shown exitingthe blowdown drum 110 in lines 102 and 103, respectively, in FIGS. 1 and2. Preferably, the blowdown separation discharges vapor-phase alkylatingagent-rich stream, and liquid phase metal salt-rich blowdown. In certainembodiments, as illustrated in FIGS. 1 and 2, the alkylating agent-richfeed stream may be delivered via line 102 directly to an aromaticalkylation reaction block 150 comprising an alkylation unit housing (orconfigured to house) the aluminosilicate zeolite catalyst. Optionally,this feed stream 102 may be heated by any suitable means between theblowdown separation and the aromatic alkylation reaction block. Theblowdown may be taken via line 103 for further treatment, discussed ingreater detail below.

The blowdown drum 110 or other separation unit may comprise a demisterpad, such as the demister pad 111 shown in FIGS. 1 and 2. The demisterpad may serve to reduce the amount of metal salt-containing liquidentrained in the alkylating agent-rich vapor stream, thereby furtherreducing the metal salt content of the vapor stream. A blowdown drum 110or other separation unit may also include any number of heat exchangersand/or condensers as necessary to effectuate separation into the(preferably vapor phase) alkylating agent-rich feed stream and the(preferably liquid phase) metal salt-rich blowdown.

In some embodiments, a blowdown drum 110 or other unit effectingseparation may be operated at conditions sufficient to create thealkylating agent-rich vapor stream and metal salt-rich blowdown, suchthat the alkylating agent-rich vapor stream comprises little or no metalsalt. In some embodiments, the discharged alkylating agent-rich streammay comprise less than 0.1 wppm metal salts. In other embodiments, thealkylating agent-rich stream may comprise less than 0.09, 0.08, 0.07,0.06, 0.05, 0.04, 0.03, 0.02, or 0.01 wppm metal salts. Operatingconditions of the blowdown drum 110 or other blowdown separations unitin various embodiments will depend at least in part upon the alkylatingagent present in the feedstock. For example, in various embodiments,operating conditions of the blowdown drum 110 may include: overheadpressure exiting the blowdown drum ranging from at least any one of 450,500, 550, 575, 600, 625, and 650 kPa(g) (gauge) to at most any one of700, 725, 750, 775, 800, 825, 850, 875, 900, 925, 950, 975, and 1000kPa(g). Any integer value between the aforementioned end points may forman upper or lower operating endpoint, as well. For instance, in someembodiments, overhead pressure exiting the blowdown drum may range fromabout 579 to about 864 kPa(g). In other embodiments, the pressure mayrange from about 535 to about 915 kPa(g), and so on. Operatingtemperatures may range from at least any one of about 100, 105, 110,115, 120, 125, 130, 135, and 140° C. to at most any one of about 120,125, 130, 135, 140, 145, 150, 155, 160, 165, and 170° C. Any integervalue between the aforementioned end points may form an upper or loweroperating endpoint, as well. For example, operating temperature mayrange from about 123 to about 135° C., or in other embodiments, fromabout 111 to about 149° C., and so on.

Conveniently, operational parameters may instead or in addition beexpressed in terms of the proportion of feedstock discharged as blowdown(that is, the percentage by weight of entering feedstock that isdischarged in the blowdown). For instance, some embodiments includedischarging blowdown comprising between about 3% to about 15% by weightof the feedstock. Other embodiments may include discharging blowdowncomprising as little as 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10% by weight ofthe feedstock, and comprising as much as 5, 6, 7, 8, 9, 10, 11, 12, 13,14, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40, 45, 50, or 55% by weight ofthe feedstock. For example, the blowdown in some embodiments maycomprise between about 5% and about 10% by weight of the feedstock, orbetween about 6% and about 9%, or between about 7% and about 35%.

In operation according to certain embodiments, the blowdown proportionmay bear a direct relationship to metal salt concentration remaining inthe discharged alkylating agent-rich stream. FIG. 4 is a graphillustrating sodium concentration in a methanol stream fed to anaromatic alkylation reactor block after blowdown separation. The data isbased upon calculations and mass balances on a blowdown separation ofmethanol feedstock having 1 ppmw sodium, wherein 0.1 wt % liquid isentrained in the vapor discharged by the blowdown drum, which is anamount of entrainment which may be encountered during typical operationof a blowdown drum with a demister tray. (In general, the finer the meshon the demister tray, and/or the thicker the demister tray, and/or thegreater the number of demister trays, the less liquid entrainment willbe encountered.) As illustrated in FIG. 4, the direct relationship inthese circumstances approximately follows the equationy=0.0915x^(−0.965), where y is metal salt concentration in thedischarged alkylating agent-rich feed stream (wppm) and x is blowdown asa percentage of the feedstock. In general, metal salt concentration inalkylating agent-rich streams discharged from blowdown separationaccording to some embodiments of the present invention may follow therelationship y=ax^(−0.965), where α is a constant directly proportionalto the feedstock's initial metal salt concentration and the wt % ofentrained liquid in the vapor.

As shown in FIG. 4, various embodiments of blowdown separation maytherefore exhibit an asymptotic approach to just under 0.01 wppm metalsalt concentration. Thus it can be seen that some preferred embodimentsmay involve discharge of blowdown comprising about 5% to about 9%, orabout 6% to about 8%, or about 7%, by weight of the feedstock. Forinstance, as FIG. 4 illustrates, when operating at 7% blowdown,obtaining just a 0.005 wppm additional reduction in metal saltconcentration requires an additional 3% of feedstock discharged toblowdown, i.e., operating so as to discharge 143% of the blowdowndischarged by operating at 7% blowdown. This could result in a sizeableincrease in operating expenses in further separations of the blowdown,and therefore may not be desired.

Further Treatment of Blowdown and/or Feedstock

Although inclusion of blowdown separation alone could in theory achievethe objective of passing feedstock to the aluminosilicate catalyst withlittle or no metal salt, it would do so at the cost of disposing asignificant proportion of the alkylating agent in the feedstock (as muchas 10% or more in some cases). This would likely result in unacceptablewaste, including harmful environmental effects and poor processeconomics. According to some embodiments, the blowdown may be divertedto a process that is not sensitive to metal salt content, such as fuelblending. However, in some instances, it may be more desirable to retainas much of the alkylating agent as possible for the aromatic alkylationprocess. In such embodiments, additional stages of blowdown separation(e.g., additional heating separation stages similar to the first, suchas additional blowdown drums in series) could resolve this problem.However, preferred embodiments include additional treatment processesthat could yield enhanced efficiency over additional blowdown separationstages. In particular embodiments, the additional treatment recoversalkylating agent that is substantially free of all metal salts, to beadded to the alkylating agent feed stream discharged from the blowdowndrum. “Substantially free” may in some embodiments mean less than 1 wppb(0.001 wppm), or in other embodiments, less than 0.1, 0.01, or 0.001 ppbmetal salts. Further, the more efficient the additional separation ofthe blowdown, the greater percentage blowdown from the blowdown drum thesystem can handle (therefore allowing the possibility of even lower saltconcentrations with minimal added cost).

As noted previously, further treatment of blowdown may compriseadditional separation processes. In some embodiments, further treatmentmay comprise any one or more of desalting and alkylating agent recovery,each of which may take place in any one or more separation stages (eachstage taking place in one or more vessels). Any suitable separationprocess may be employed at each stage, including one or more of sorption(e.g., adsorption, absorption, ion exchange, cold clay treating, and thelike), stripping, cascading, distillation, membrane separation, and thelike.

Returning to the example embodiment of FIG. 1, blowdown may be separatedin a two-stage process comprising: (1) desalting by separation into analkylating agent-enriched liquid stream and a first metal salt-richliquid effluent, followed by (2) alkylating agent recovery by separationof the alkylating agent-enriched liquid stream into an alkylatingagent-enriched vapor stream and a second metal salt-rich liquideffluent. As shown in FIG. 1, the first separation stage (1) takes placein desalting section 129, and the second separation stage (2) takesplace in alkylating agent recovery section 139. As noted, eachseparation stage may take place in one or more sub-stages according tovarious other embodiments not illustrated in FIG. 1. Specifically,according to FIG. 1, liquid blowdown is delivered via line 103 to thedesalting section 129. The desalting section 129 may comprise adesalting column 120 of any suitable type (adsorption, cascade,absorption, distillation, fractionation, etc.). As shown in theembodiment of FIG. 1, desalting column 120 is a sorption column carryingout sorption (specifically absorption) of metal salts from the liquidblowdown using a water wash delivered via line 128. Any known unit foreffecting a liquid-liquid or gas-liquid sorption (or other suitableseparation process as noted previously) may be employed as a desaltingcolumn 120, such as liquid-liquid or vapor-liquid cascadeconfigurations. Preferably, the desalting column 120 further comprisesheating means 121 (e.g., a heat exchanger such as a reboiler) coupled toa bottom portion of the desalting column 120 to effect or aid inrecovery of alkylating agent from the blowdown. In such configurations,the desalting column may discharge an alkylating agent-rich effluent(preferably liquid phase) in line 104, which optionally may be cooled(e.g., by condenser 122) and collection (e.g., in tank 123) before beingpumped via lines 124 and 125 to alkylating agent recovery section 139.Optionally, a portion of the condensed effluent may additionally berecycled to a top portion of the desalting column 120, as shown by line105 in FIG. 1. Any other separation unit and/or process suitable forseparation into an overhead vapor phase and bottom liquid phase may beused, or alternatively, any separation unit and/or process suitable forseparation into overhead alkylating agent-rich effluent and bottomswater-rich stream may be used as the desalting column (e.g.,distillation and/or fractionation).

The desalting column 120 may operate under any suitable conditions toeffect the above-described separation. In some instances, suitableconditions may depend at least in part upon the type of separationprocess employed in the desalting column 120. Preferably, both thealkylating agent-rich effluent and the metal salt-rich effluent leavingthe column (e.g., via lines 104 and 106 in FIG. 1) are liquid phase, andaccordingly, preferred operation of the desalting column 120 accordinglyyields both streams in the liquid phase. Operating conditions of thedesalting column may depend upon the alkylating agent present in theinitial feedstock. For example, in embodiments in which the feedstockcomprises methanol, the operating conditions of the desalting column 120may include: exit overhead pressure ranging from at least any one ofabout 500, 525, 550, 575, 600, 625, 650, 675, 700, and 725 kPa(g)(gauge) to at most any one of about 750, 775, 800, 825, 850, 875, 900,925, 950, 975, and 1000 kPa(g). Any integer value between theaforementioned end points may form an upper or lower operating endpoint,as well. For instance, in some embodiments, overhead pressure exitingthe blowdown drum may range from about 623 to about 898 kPa(g). In otherembodiments, the pressure may range from about 595 to about 944 kPa(g),and so on. Operating temperatures may range from at least any one ofabout 100, 105, 110, 115, 120, 125, 130, 135, and 140° C. to at most anyone of about 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, and170° C. Any integer value between the aforementioned end points may forman upper or lower operating endpoint, as well. For example, operatingtemperature may range from about 119° C. to about 130° C., or in otherembodiments, from about 107 to about 149° C., and so on.

Moreover, in some embodiments, regardless of the identity of thealkylating agent, the alkylating agent-rich effluent may comprise about70 to about 99% by weight of the liquid blowdown entering the desaltingsection 129. In various other embodiments, the alkylating agent-richeffluent may comprise at least about 70, 75, 80, 85, 90, 91, 92, 93, 94,95, 96, 97, 98, or 99% by weight of the liquid blowdown, and at mostabout 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 99.5, or 99.9% byweight of the liquid blowdown. Further, the alkylating agent-richeffluent should comprise, in various embodiments, at least 95, 96, 97,98, or 99% by weight of the alkylating agent entering the desaltingcolumn 120 in the liquid bottoms. In other embodiments, substantiallyall (e.g., 99.99, 99.999, 99.9999, 99.99999% or more, by weight) of thealkylating agent from the liquid bottoms exits the desalting column 120in the alkylating agent-rich effluent via line 104.

In addition, the metal salt-rich liquid discharge (comprising at leastmetal salt and in applicable embodiments water, such as most or all ofany water wash used in sorption or the like) leaving as the desaltingcolumn 120 bottoms in line 106 may in some embodiments comprise no morethan 5, 4, 3, 2, 1, 0.5, 0.05, or 0.005% alkylating agent, by weight ofthe liquid discharge. Expressed in other terms, the liquid dischargeshould in various embodiments contain no more than 1%, preferably nomore than 0.1, 0.01, 0.001, 0.0001, or 0.00001% by weight of thealkylating agent that entered the desalting column 120 in the liquidbottoms. That is, the far majority (and preferably substantially all) ofthe alkylating agent that enters the desalting column should bedischarged in the overhead comprising the alkylating agent-richeffluent. Further, the metal-salt rich liquid discharged in line 106 invarious embodiments comprises at least about 90, preferably more than95, 96, 97, 98, 99, 99.9, or 99.99% by weight of the metal salts thatwere present in the inlet liquid blowdown in line 103.

As noted, the alkylating agent-rich effluent is discharged as overheadin line 104 and delivered to the alkylating agent recovery section 139by line 125. The alkylating agent recovery section 139 as shown includesa stripper unit 130 for extracting an alkylating agent feed streamsubstantially free of metal salts. The alkylating agent feed stream maybe provided to an alkylation reaction block 150, where alkylation suchas methylation of toluene and/or benzene may take place in the presenceof aluminosilicate zeolite catalyst. In preferred embodiments, thestripper unit 130 may also strip at least a portion of alkylating agentfrom a recovery stream recycled from the alkylation reaction block 150,as shown by recycle line 126 in FIG. 1. The stripper unit 130 may insome embodiments be a distillation column, fractionator, or any likeseparations unit suitable for extracting an alkylating agent feed streamfrom both the alkylating agent-rich effluent and the recovery stream. Itmay optionally include heating means 131 such as a heat exchanger and/orreboiler coupled to a bottom portion of the stripper unit 130.

In addition to the alkylating agent feed stream, the stripper unit 130also ejects liquid discharge, preferably as the bottoms from the column,as shown in line 137 of FIG. 1. In certain preferred embodiments, atleast a portion of the stripper unit bottoms in line 137 may be recycledto the desalting section 129 via line 128, where it may be employed inthe separation process of that desalting section 129 (e.g., as the waterwash in a sorption and/or cascade process).

Further, in certain embodiments, the stripper unit 130 furthermoreejects a sidestream in line 136 from a middle portion of the unit, thesidestream comprising steam and substantially no metal salts. Thesidestream further may comprise substantially no alkylating agent (e.g.,less than 100 ppm, less than 10 ppm, or less than 1 ppm, in variousembodiments). The sidestream therefore may, in some embodiments, beadded to a fresh aromatic feed 151 entering the alkylation reactor block150. The addition of steam sidestream to the fresh aromatic feed raisesthe temperature to the high operating temperatures required for aromaticalkylation according to some embodiments (e.g., temperatures rangingfrom any one of about 400, 450, 475, 500, 525, and 550° C. up to any oneof about 500, 525, 550, 575, 600, 625, 650, 675, and 700° C.). Theaddition of steam sidestream to fresh aromatic feed further reduces thepartial pressure of the aromatic compounds fed to an aromatic alkylationreaction unit, which helps prevent coking. Because processes, systems,and apparatus of certain embodiments eliminate the presence of metalsalts in this sidestream, the sidestream may provide the above benefitswithout the downside of metal salts contacting the aluminosilicatezeolite catalyst in an aromatic alkylation reaction unit.

The stripper unit 130 may operate under conditions sufficient todischarge at least the alkylating agent-rich vapor feed stream, themetal salt-rich liquid discharge, and the sidestream. In someembodiments, the alkylating agent-rich vapor feed stream comprises atleast 20% by weight (of the vapor feed stream) alkylating agent. In someembodiments, the alkylating agent-rich vapor feed stream comprises atany one of 5, 10, 15, 20, 25, 27, 30, 35, and 40% by weight alkylatingagent, and at most any one of 10, 15, 20, 25, 27, 30, 35, 40, 45, 50,55, 60, 65, and 70% by weight alkylating agent. The balance may comprisemostly water (steam), with minor amounts (less than 5, 4, 3, 2, 1 or0.5% by weight of total feed stream in various embodiments) of tracehydrocarbon impurities which may be present, e.g., due to an alkylatingagent recovery recycle feed from a reaction block to the dividing-wallcolumn 500. Further, the alkylating agent-rich feed stream dischargedfrom the stripper section 139 should be substantially free of metalsalts when carried out according to preferred embodiments. As noted, thealkylating agent-rich vapor feed stream further comprises water and/orsteam. In certain embodiments, the alkylating agent-rich vapor feedstream comprises at least 50% by weight water and/or steam. Thealkylating agent-rich vapor feed stream may comprise at least any one ofabout 30, 35, 40, 45, 50, 55, 60, 65, 70, 73, 75, 80, 85, or 90% byweight water (steam), and at most any one of about 60, 65, 70, 73, 75,80, 85, 90, and 95% by weight water (steam). The presence of sufficientamounts of steam (such as the amounts disclosed herein) in thealkylating agent-rich vapor feed stream in an aromatic alkylationreaction using methylating agents as alkylating agents advantageouslymay improve selectivity to alkylated aromatics (such as xylenes, inparticular para-xylene), as opposed to production of products which areundesired in certain embodiments, such as olefins. Advantageously, then,separations operations may at this point focus primarily on reducingentrained liquid (and thereby entrained metal salt) present in theoverhead vapor stream in order to provide an alkylating agent-rich vaporfeed stream comprising substantially no metal salts (i.e., less than 1,0.1, or 0.01 ppm by weight in various embodiments).

The liquid discharge in the bottoms of stripper unit 130 preferablyincludes no more than 1, 0.1, 0.01, or 0.001 ppb by weight alkylatingagent. Preferably, the liquid discharge contains substantially all (invarious embodiments, at least 99.9, 99.99, 99.999, or 99.9999% byweight) of the metal salts in the alkylating agent-rich liquid effluentfed to the stripper unit 130 from the desalting column 120.

In summary, recall that (i) the metal salt-rich liquid effluentdischarged via line 106 from the desalting column 120 comprises lessthan 1%, preferably no more than 0.1, 0.01, 0.001, 0.0001, or 0.00001%by weight of the alkylating agent that entered the desalting column 120in the liquid bottoms; and further that (ii) the metal salt-rich liquiddischarge purged from the stripper unit 130 via line 137 comprises lessthan 1, 0.1, 0.01, or 0.001 ppb by weight alkylating agent in variousembodiments, it can be seen that the further treatment of the blowdown(e.g., in desalting section 129 and alkylating agent recovery section139) results in loss of substantially none of the alkylating agentoriginally present in the liquid blowdown (i.e., less than 1, 0.1, or0.01 ppm by weight in various embodiments). This indicates thatsubstantially all of the alkylating agent present (e.g., all but 1 ppm,or 99.9999% or more) in the liquid blowdown is recovered in the furthertreatment according to certain embodiments, while being substantiallyfree of metal salts. Thus, some embodiments provide for furthertreatment of blowdown resulting in recovery of a feed stream comprisingsubstantially all alkylating agent in the liquid blowdown, whilecomprising substantially none of the metal salt from the liquidblowdown. According to such embodiments, then, the only source of metalsalt fed to the aluminosilicate zeolite catalyst is in the firstalkylating agent-rich stream discharged from the blowdown drum.

Dividing-Wall Column

In certain embodiments, as noted previously, the further blowdowntreatment (e.g., desalting and alkylating agent recovery, such as bysorption and stripping) may take place substantially in a single vessel,which may be a dividing-wall unit. Processes according to suchembodiments may be carried out by a system configured as illustrated inFIG. 3, wherein the liquid blowdown is fed via line 103 to a singledividing-wall unit 230, which serves the combined functions of thedesalting column 120 and the stripper unit 130 of FIG. 1. In particular,according to some embodiments, the dividing-wall unit 230 discharges atleast an alkylating agent-rich feed stream (e.g., via line 235 in FIG.2); a sidestream (line 236); and a metal salt-rich liquid discharge(line 237), corresponding to each stream respectively discharged fromthe stripper unit 130 in lines 135, 136, and 137 of embodimentsaccording to the example of FIG. 1. The dividing-wall unit 230 mayoperate within any of the temperature and pressure conditions previouslydescribed with respect to the desalting column 120.

FIG. 3 illustrates a further embodiment utilizing a dividing-wall unit230, wherein the initial blowdown separation of the feedstock mayoptionally be skipped. Thus, feedstock comprising alkylating agent andmetal salt is fed directly via line 301 to the dividing-wall unit 230.In such embodiments, the dividing-wall unit operation may optionallyinvolve additional trays (actual or theoretical), or operation atdecreased temperature to ensure the absence or near-absence of metalsalts in the overhead. In other embodiments, however, the dividing-wallunit 230 may be operated in substantially the same manner as inembodiments that include other separations such as initial blowdown.

Certain embodiments wherein feedstock is delivered directly to thedividing-wall unit 230 may take advantage of the need for steam in thealkylating agent-rich feed stream discharged in overhead line 235according to some embodiments, as discussed previously. For instance,use of a dividing-wall unit may simplify the separation of metal saltsfrom alkylating agent feeds by virtue of being able to operate at highertemperatures (e.g., to ensure presence of both steam and alkylatingagent in the overhead), thus ensuring that no metal salts are present inthe overhead feed stream in line 235, while also avoiding the need forfurther separations of steam from that overhead feed stream (as opposedto processes such as olefin-synthesis from alkylating agents, whereinoperation at such conditions would necessitate separation of the steamin the feed stream, among other further modifications).

FIG. 5 is a schematic illustrating an example dividing-wall unitaccording to some embodiments. The embodiment shown in FIG. 5illustrates a dividing-wall column 500 comprising a desalting zone orportion 505 embedded among an alkylating agent recovery zone or portion510, which in the embodiment of FIG. 4 comprises a plurality of trays550 (in other embodiments, the alkylating agent recovery zone or portion510 may comprise packing, a fixed, fluid, or fluidized fluid bed, or thelike). The number of trays 550 shown in FIG. 5 is not necessarilyrepresentative; for instance, more trays 550 may be present above,below, or both above and below the desalting zone or portion 505. Inparticular, more trays 550 may be present above the desalting shown inembodiments wherein it is desired to eject a sidestream via line 525comprising substantially no alkylating agent (discussed later).

The column 500 additionally includes heating means such as a reboiler520 fluidically coupled to (i.e., in fluid communication with) a bottomportion of the column 500 so as to provide reflux of the bottomsdischarged from the column via line 532. The column 500 is preferablyoperated at a temperature sufficient to create a vapor phase overhead ofalkylating agent-rich feed stream, which may be discharged via line 531to, e.g., an aromatic alkylation reaction block. As vapor phasecomprising alkylating agent rises through the trays 550 of thealkylating agent recovery zone or portion 510, it passes through theembedded desalting zone or portion 505, which as shown in FIG. 5 may bea dis-entrainment zone, wherein water wash 511 is provided via line 515to reduce entrainment of liquid phase within the vapor phase passingthrough the desalting zone. The reduction of entrainment may in someembodiments occur via a sorption process (e.g., liquid-gas absorption),or by any other suitable means of reducing entrained liquid in the vaporphase. The wash contacts at least a portion of the vapor phase andeffects removal (e.g., adsorptive and/or absorptive removal, and/orcondensation) of liquid (including metal salts dissolved therein) fromthe vapor phase as the vapor phase travels up the column 500, whileliquid phase comprising metal salts falls to the bottom of the column tobe discharged in the bottoms via line 532. The desalting zone or portion505 of some embodiments may be arranged as shown in FIG. 5, such thatliquid phase comprising metal salts in the alkylating agent recoveryportion falls around, rather than through, the desalting zone or portion505, as shown by liquid phase path 555 in FIG. 5. Further, the desaltingzone or portion 505 may include packing, such as crinkly wire meshscreen (CWMS), trays, a fluid, fluidized, or fixed bed, or the like, toenhance the embedded separation taking place therein. For instance,where adsorptive separation takes place, the desalting zone or portion505 may include a fluid, fluidized, or fixed bed.

The alkylating agent-rich feed stream, as noted, is discharged as vaporvia line 531. In various embodiments, this feed stream may comprise atleast any one of 5, 10, 15, 20, 25, 27, 30, 35, and 40% by weightalkylating agent, and at most any one of 10, 15, 20, 25, 27, 30, 35, 40,45, 50, 55, 60, 65, and 70% by weight alkylating agent. The balance maycomprise mostly water (steam), with minor amounts (less than 5, 4, 3, 2,1 or 0.5% by weight of total feed stream in various embodiments) oftrace hydrocarbon impurities which may be present, e.g., due to analkylating agent recycle feed from a reaction block to the dividing-wallcolumn 500 (delivering, among other things, unreacted alkylating agentfrom an aromatic alkylation, which stream may comprise trace hydrocarbonimpurities).

The metal salt-rich bottoms (discharged via line 532) preferablycontains less than 1% by weight alkylating agent and more than 99% byweight water (the water including metal salts). In certain embodiments,the bottoms comprise less than 1 ppb, 0.1 ppb, 0.01 ppb, or 0.001 ppb byweight alkylating agent. Preferably, the bottoms contains substantiallyall (in various embodiments, at least 99.9, 99.99, 99.999, or 99.9999%by weight) of the metal salts originally present in the blowdown orfeedstock fed to the dividing-wall column via line 501.

In addition, a vapor phase sidestream is drawn off a middle portion ofthe column 500 via line 525, e.g. via one of the trays above thedesalting zone 505. The sidestream comprises mostly water (steam),preferably at least 80, 90, 95, 98, 99, 99.5, 99.9, or 99.99% by weightwater in various embodiments. The sidestream comprises substantially nometal salts, and may comprise at most 0.01, 0.1, 1, 2, 5, 10, or 20%alkylating agent in various embodiments. The sidestream may, like thesidestream of some previously described embodiments, be mixed witharomatic feed to an aromatic alkylation reaction block.

In summary, as with embodiments in which further treatment of blowdownis performed in a desalting step and then an alkylating agent recoverystep, treatment by dividing-wall column (of either blowdown orfeedstock) may result in recovery of substantially all of the alkylatingagent in the blowdown or feedstock initially fed to the dividing wallcolumn, with such recovered alkylating agent stream containingsubstantially no metal salts. Further, treatment processes employing adividing-wall column comprising a desalting zone, as with embodimentsjust described, may provide the additional advantage of desalting anyalkylating agent fed to the dividing-wall column in an alkylating agentrecycle stream (e.g., recycled from an aromatic alkylation reactionblock). This advantageously may remove at least some, if not all, metalsalts that may be present in the recycle stream (due, e.g., to tracemetal salts fed to the reactor by other means, or from an alkylatingagent-rich vapor stream discharged from a blowdown separation, inembodiments where such separation is employed).

Thus, in accordance with the processes and systems of variousembodiments just described, some embodiments of the present inventionmay provide a dividing-wall separations unit comprising: (i) analkylating agent recovery zone or portion comprising a plurality oftrays and (ii) a desalting zone or portion disposed among the alkylatingagent recovery zone or portion. The alkylating agent recovery zone orportion of some embodiments may comprise a plurality of trays, such thatthe desalting zone or portion is disposed between at least two of theplurality of trays. According to certain embodiments, the alkylatingagent recovery zone may be a stripping zone, which may be configured toeffect stripping of an alkylating agent from a liquid in the alkylatingagent recovery zone. In applicable embodiments, the trays may bedisposed substantially horizontally within the dividing-wall unit andarranged in a vertical stack, with space between each tray, as in adistillation column. The desalting portion may be disposed between twoof the trays. The two trays between which the desalting portion isdisposed may in some embodiments be spaced farther apart than any othertwo trays in the alkylating agent recovery portion. In arrangementsaccording to embodiments such as that shown in FIG. 5, the alkylatingagent recovery portion of the dividing-wall separations unit maytherefore in essence be split into two sub-portions by the desaltingportion. As also shown in FIG. 5, the desalting portion may comprise adis-entrainment zone, which may operate according to the abovedescription of the embedded desalting zone or portion 505 (e.g., viasorption and/or condensation for removal of entrained liquid from thevapor phase). The dis-entrainment zone may comprise a demisting pad(e.g., to further aid in removal of liquids entrained in a vaporcontacting the demisting pad).

The dividing-wall separations unit may further comprise any one or moreof: (i) a feed inlet; (ii) a recycle inlet; (iii) an overhead outletdisposed at a top portion of the dividing-wall separations unit; (iv) asidestream outlet; and (v) a bottoms outlet disposed at a bottom portionof the dividing-wall separations unit. The sidestream outlet may bedisposed on the dividing-wall unit above the desalting portion and belowthe overhead outlet, as shown in FIG. 5. In embodiments wherein thedesalting portion comprises a dis-entrainment zone, the dividing-wallseparations unit may further comprise (vi) a wash inlet (exemplified bywater-wash line 515 in FIG. 5). The (i) feed inlet may be configured toreceive liquid alkylating agent feedstock and/or liquid blowdown; the(ii) recycle inlet may be in fluid communication with an aromaticalkylation reaction unit and configured to receive a stream comprisingunreacted alkylating agent from the aromatic alkylation reaction unit;the (iii) overhead outlet may be configured to discharge an overheadstream comprising alkylating agent-rich feed stream and/or steam, and itmay further be in fluid communication with the aromatic alkylationreaction unit (e.g., so as to deliver the alkylating agent-rich feedstream and/or steam to the reaction unit); the (iv) sidestream outletmay be configured to discharge steam, and in some embodiments,substantially no alkylating agent, and may further be combined with anaromatic feed stream and be in fluid communication with the aromaticalkylation reaction unit; the (v) bottoms outlet may be configured todischarge a liquid bottoms comprising substantially all metal saltspresent in the feed inlet stream; and the (vi) wash inlet may beconfigured to receive a wash such as a liquid sorbent to be dispersed(e.g., sprayed, cascaded, or otherwise discharged) into the desaltingportion of the dividing-wall separations unit.

In other embodiments, the dividing-wall separations unit may be arrangedsuch that the desalting portion is not embedded within the alkylatingagent recovery portion. Instead, for instance, the desalting portion andalkylating agent recovery portion may be arranged beside each other in adividing-wall column, with a vertical dividing wall separating the two(as stylized, e.g., by the simplified diagram of the dividing-wallcolumn 230 of FIGS. 2 and 3). In such embodiments, each of the desaltingportion and the alkylating agent recovery portion may operatesubstantially as the separate units would operate. That is, for example,the desalting portion may function as a desalting unit such as adesalting column, discussed previously, with the overhead alkylatingagent-rich effluent delivered over the dividing wall to the alkylatingagent recovery portion and the bottoms liquid discharge delivered to thebottom portion of the alkylating agent recovery portion through oraround a bottom portion of the dividing wall. Further, in suchembodiments, the alkylating agent recovery portion may operatesubstantially as a separate alkylating agent recovery unit (e.g., astripper unit, fractionation or distillation unit, or the like,discussed previously).

Aromatic Alkylation

As noted throughout, various embodiments may further provide forreacting at least part of the alkylating agent-rich feed stream(s)produced by the various treatment methods and systems with one or morearomatic compounds in the presence of an aluminosilicate zeolitecatalyst. Such reaction may take place in an aromatic alkylationreaction block. As also noted, aromatic alkylation can be carried outwith any known alkylating agent, but preferred alkylation includesmethylation of benzene and/or toluene, and preferred methylation agentsinclude methanol, and/or a mixture of carbon monoxide and hydrogen,and/or dimethyl ether.

Suitable conditions for the methylation reaction include a temperaturefrom 350 to 700° C., such as from 500 to 600° C., a pressure of from 100and 7000 kPa absolute, a weight hourly space velocity of from 0.5 to1000 hr⁻¹, and a molar ratio of toluene to methanol (in the reactorcharge) of at least about 0.2, e.g., from about 0.2 to about 20. Theprocess may suitably be carried out in fixed, moving, or fluid catalystbeds. If it is desired to continuously control the extent of cokeloading, moving or fluid bed configurations are preferred. With movingor fluid bed configurations, the extent of coke loading can becontrolled by varying the severity and/or the frequency of continuousoxidative regeneration in a catalyst regenerator. One example of asuitable fluidized bed process for methylating toluene includes stagedinjection of the methylating agent at one or more locations downstreamof the toluene feed location. Such a process in described in U.S. Pat.No. 6,642,426, the entire contents of which are incorporated herein byreference.

Using various of the present processes, toluene can be alkylated withmethanol so as to produce para-xylene at a selectivity of at least about75 wt % (based on total C₈ aromatic product) at a per-pass aromaticconversion of at least about 15 wt % and a trimethylbenzene productionlevel less than 1 wt %. Unreacted toluene and methylating agent and aportion of the water by-product may be routed to an alkylating agentrecovery zone as hereinabove described (with or without separation ofmethylating agent from toluene), and/or they can be recycled to themethylation reactor. The C₈ fraction is routed to a para-xyleneseparation section, which typically operates by fractionalcrystallization or by selective adsorption or both to recover apara-xylene product stream from the alkylation effluent and leave apara-xylene-depleted stream containing mainly C₇ and C₈ hydrocarbons.

EXAMPLES Example 1

This simulated example illustrates the effect of performing a blowdownseparation on methanol feedstock with varying levels of metal salt(here, sodium salt) contamination, based upon calculations and massbalances for methanol feedstock of 1 ppmw sodium and for methanolfeedstock of 0.32 ppmw sodium, using blowdown operation resulting in 0,3, and 7% liquid blowdown, by weight of feedstock. Sodium buildup on thealuminosilicate zeolite catalyst was calculated for long-term operationsand found to increase until asymptotically approaching a maximum buildupconcentration after 60 months of operation. As can be seen in Tables 1aand 1b below, exponential improvement in prevention of sodium buildup isachieved from 0 to 3% blowdown, but improvement becomes marginally lower(virtually linear) for 3 to 7% blowdown, indicating a potentiallysignificant loss of efficiency in operating to achieve lower evenconcentrations of sodium.

TABLE 1a Sodium Concentrations for 1 ppmw Sodium in Methanol FeedstockNa in Reaction Feed Na Buildup on Blowdown % (ppmw) Catalyst (ppmw) 01.000 2041.000 3 0.034 69.390 7 0.015 30.620

TABLE 1b Sodium Concentrations for 0.32 ppmw Sodium in MethanolFeedstock Na in Reaction Feed Na Buildup on Blowdown % (ppmw) Catalyst(ppmw) 0 0.32 653.120 3 0.011 22.210 7 0.005 9.800

Example 2

This example illustrates a simulated system employing a processaccording to various embodiments, the system configured substantially asshown in FIG. 6. Simulations according to this example were carried outusing SimSci™ PRO/II™ process simulation software, commerciallyavailable from Invensys, Inc. The system includes a liquid methanolinlet feed 601 (which may be considered as either feedstock or blowdownfor purposes of this example) delivered to desalting column 610. Thedesalting column 610 in this example is a distillation column. Thedesalting column discharges methanol-rich overhead in line 602 andbottoms enriched in water (and dissolved metal salts) in line 603. Thedesalting column 610 as simulated included partial recycle of thebottoms via a reboiler, and partial recycle of the overhead via acondenser, which are omitted from FIG. 6 for simplicity. Overhead 602 iscombined with methanol recycle stream 604 (e.g., from a reactor formethylation of toluene, not shown), forming stream 605 fed into the topof methanol stripper 620. Methanol stripper 620 discharges overhead 606comprising water, methanol, and trace hydrocarbons in 100% vapor phase(i.e., having no entrained liquid and therefore no dissolved metalsalts). Methanol stripper 620 further discharges sidestream 607 andbottoms 608 (part of which was reboiled and recycled to methanolstripper 620 as simulated, again omitted from FIG. 6 for simplicity).Table 2 illustrates the composition of each stream 601-608, showingsuccessful operation of the simulated system in accordance with variousof the embodiments described herein.

TABLE 2 Stream Compositions of Example 2 Feed 601 602 603 604 605 606607 608 Phase Liquid Liquid Liquid Liquid Liquid Vapor Vapor Mixed Temp.(° C.) 110.5 119.2 160.6 107.6 108.8 150.2 160.6 160.9 Kg/hr 3,287.43,252.1 35.3 57,302.3 6,0554.4 23,520.8 7,985.7 29,047.9 Water* 1.410.34 99.995 89.59 84.79 60.89 99.91 99.99 Methanol* 98.59 99.66 50 ppm8.63 13.52 34.80  .23 ppm 9.8 ppt⁺ Toluene* 0 0 0 0.13 0.12 0.31 0 0Misc. other 0 0 0 1.66% 1.57% 4.00% 877 ppm 108 ppm  Hydrocarbon**Compositions are in wt % unless otherwise noted ⁺ppt = parts pertrillion

As shown in Table 2, almost no methanol is discharged from the system(i.e., in lines 603 and lines 608), while the methanol feed for deliveryto alkylation reaction (line 606) includes no entrained liquid andtherefore no metal salts.

While the present invention has been described and illustrated byreference to particular embodiments, those of ordinary skill in the artwill appreciate that the invention lends itself to variations notnecessarily illustrated herein. For this reason, then, reference shouldbe made solely to the appended claims for purposes of determining thetrue scope of the present invention.

1. A process for producing para-xylene, the process comprising: (a)separating a feedstock comprising an alkylating agent and a metal saltinto at least an alkylating agent-rich vapor stream and a metalsalt-enriched liquid blowdown comprising alkylating agent and metalsalt; and (b) reacting at least a portion of the alkylating agent-richvapor stream with one or more aromatic compounds in the presence of analuminosilicate zeolite catalyst under conditions sufficient to yieldpara-xylene.
 2. The process of claim 1, further comprising: (c) treatingthe metal salt-enriched liquid blowdown so as to provide at least oneadditional alkylating agent-rich vapor stream and a metal salt-richliquid discharge; and (d) reacting at least a portion of the at leastone additional alkylating agent-rich vapor stream with one or morearomatic compounds in the presence of the aluminosilicate zeolitecatalyst under conditions sufficient to yield para-xylene.
 3. Theprocess of claim 2, wherein treating the metal salt-enriched liquidblowdown in step (c) comprises: (c1) sorbing at least a portion of themetal salt from the metal salt-enriched liquid blowdown so as to form atleast an alkylating agent-rich liquid stream and a first metal salt-richliquid effluent; and (c2) separating the alkylating agent-rich liquidstream into the at least one additional alkylating agent-rich vaporstream and a second metal salt-rich liquid effluent; wherein the metalsalt-rich liquid discharge comprises at least a portion of each of thefirst and the second metal salt-rich liquid effluents.
 4. The process ofclaim 3, wherein at least a portion of the second metal salt-rich liquideffluent is recycled and used to sorb the portion of the metal salt-richliquid blowdown.
 5. The process of claim 2, wherein treating the metalsalt-rich liquid blowdown in step (c) comprises separating the metalsalt-enriched liquid blowdown in a dividing-wall column comprising adesalting zone and an alkylating agent recovery zone, so as to form theat least one additional alkylating agent-rich vapor stream, a sidestream, and the metal salt-rich liquid discharge.
 6. The process ofclaim 2, wherein the metal salt-rich liquid discharge comprises 99.9% ormore by weight of the metal salt of the feedstock.
 7. The process ofclaim 1, wherein the alkylating agent comprises methanol, dimethylether, or a combination thereof.
 8. The process of claim 1, wherein themetal salt comprises sodium.
 9. The process of claim 1, wherein thealuminosilicate zeolite catalyst comprises a porous crystalline materialhaving a Diffusion Parameter for 2,2 dimethylbutane of about 0.1-15sec⁻¹ when measured at a temperature of 120° C. and a 2,2 dimethylbutanepressure of 60 torr (8 kPa).
 10. The process of claim 9, wherein theporous crystalline material comprises ZSM-5 that has undergone priortreatment with steam at a temperature of at least 950° C.
 11. Theprocess of claim 1, wherein the conditions of the (b) reacting are suchthat selectivity to para-xylene is at least 75%.
 12. The process ofclaim 1, wherein the liquid blowdown comprises about 5 to about 10% byweight of the feedstock.
 13. The process of claim 12, wherein the liquidblowdown comprises about 7% by weight of the feedstock.
 14. A processfor treating feedstock, the process comprising: providing a feedstockcomprising alkylating agent and metal salt to a dividing-wall columncomprising a desalting zone embedded within an alkylating agent recoveryzone; and using the dividing-wall column, separating the feedstock intoat least (i) an alkylating agent-rich overhead, (ii) a sidestream, and(iii) a metal salt-rich liquid discharge comprising 99.9% or more byweight of the metal salt of the feedstock.
 15. The process of claim 14,wherein the alkylating agent-rich overhead comprises 99.9% or more byweight of the alkylating agent of the feedstock.
 16. The process ofclaim 14, wherein the metal salt-rich liquid discharge comprises 99.9%or more by weight of the metal salt of the feedstock.
 17. The process ofclaim 14, wherein the feedstock is not subjected to a separationsprocess prior to providing the feedstock to the dividing-wall column.18. The process of claim 14, further comprising alkylating one or morearomatic compounds with at least a portion of the alkylating agent-richoverhead in the presence of an aluminosilicate zeolite catalyst.
 19. Theprocess of claim 18, wherein the alkylating agent is selected from thegroup consisting of methanol, dimethyl ether, and a combination thereof;wherein the one or more aromatic compounds comprise one or both oftoluene and benzene; and further wherein alkylating the one or morearomatic compounds yields para-xylene.
 20. A system for treatingfeedstock, the system comprising: a dividing-wall column comprising: (i)a feed inlet configured to receive a feed comprising liquid phasealkylating agent and one or more metal salts; (ii) a wash inletconfigured to receive a wash stream; (iii) an alkylating agent recoveryzone, the alkylating agent recovery zone being configured to effectseparation of an alkylating agent from a liquid in the alkylating agentrecovery zone; (iv) a desalting zone disposed among the alkylating agentrecovery zone, the desalting zone comprising a dis-entrainment zoneconfigured to receive the wash stream via the wash inlet and spray atleast a portion of the wash stream over a vapor in the desalting zone soas to effect separation of an entrained liquid comprising one or moremetal salts from the vapor; (v) a first outlet disposed at a top portionof the column, and configured to discharge a vapor-phase overhead fromthe dividing-wall column; (vi) a second outlet disposed on the columnabove the desalting zone and below the first outlet, the second outletconfigured to discharge a vapor-phase sidestream from the dividing-wallcolumn; and (vii) a third outlet disposed at a bottom portion of thecolumn below the desalting zone and configured to discharge aliquid-phase bottoms stream from the dividing-wall column.
 21. Thesystem of claim 20 further comprising a blowdown drum in fluidcommunication with the dividing-wall column via the feed inlet, theblowdown drum configured to provide a liquid blowdown to thedividing-wall column via the feed inlet.
 22. The system of claim 20further comprising an aromatic alkylation unit in fluid communicationwith the dividing-wall column via the first outlet, and configured toreceive at least a portion of the overhead from the dividing-wallcolumn.
 23. The system of claim 22, wherein the aromatic alkylation unitis further in fluid communication with the dividing-wall column via thesecond outlet, and is further configured to receive at least a portionof the sidestream from the dividing-wall column.
 24. The system of claim20, wherein the alkylating agent recovery zone comprises a plurality oftrays, and further wherein the desalting zone is disposed between two ofthe plurality of trays of the alkylating agent recovery zone.
 25. Thesystem of claim 20, wherein the dis-entrainment zone further comprises ademisting pad.